The present invention relates generally to circuit protection devices in electrical distribution systems and, more particularly, to an arcing fault detection system connected to a secondary line of a current transformer in an industrial electrical distribution system.
Electrical systems in residential, commercial and industrial applications usually include a panelboard or load center for receiving electrical power from a utility source. Depending on the application, either of several types of panelboards may be used. For instance, one common type of panelboard is a one phase, three wire system adapted to receive power from the utility source via two line bus bars and neutral bus bars. Another common type of panelboard, typically used in industrial applications, is a three phase, four wire system adapted to receive power from the utility source via three phase bus bars and a neutral bus. The electrical power is delivered from the panelboard to designated branch circuits through line and neutral conductors supplying one or more loads. The line conductors are electrically connected to the line or phase buses and the neutral conductors are electrically connected to the neutral bus.
Typically, various types of protective devices are either mounted to the bus bars of the panelboard or connected to the line and/or neutral conductors of designated branch circuits to protect the electrical distribution system from hazardous electrical conditions and reduce the risk of injury, damage or fires. An arcing fault detector is one type of protective device that may be utilized to protect the electrical distribution system from arcing faults, which occur when electric current xe2x80x9carcsxe2x80x9d or flows through ionized gas between two ends of a broken conductor, between two conductors supplying a load, or between a conductor and ground. Arcing faults typically result from corroded, worn or aged wiring or insulation, loose connections, wiring damaged by nails or staples through the insulation, or electrical stress caused by repeated overloading, lightning strikes, etc. The presence of an arcing fault creates a significant fire hazard because it generates heat which may ignite the conductor insulation and adjacent combustible materials.
There are several systems known in the art for detecting arcing faults. Generally, these systems utilize a sensing coil around the line conductor of a branch circuit for sensing characteristics of current flowing in the line conductor, then process the information to determine whether the sensed data represents the occurrence of an arcing fault. If an arcing fault is detected, the branch circuit is xe2x80x9ctrippedxe2x80x9d open to stop current from flowing in the branch circuit. Alternatively, the arcing fault detection system may sense line voltage rather than line current.
The sensing coil may be electrically coupled to the line conductor in any of several ways known in the art. For instance, the sensing coil may be incorporated within a standard circuit breaker which is mounted to the line or phase bus in a panelboard. The tripping mechanism associated with the circuit breaker may then be used to interrupt the electrical circuit in response to arcing faults as well as overloads, short circuits and/or ground faults. Sensing coils may also be coupled directly to the line bus, phase bus or line conductor of a designated branch circuit, or may be contained within separate arcing fault detector modules connected to the line conductors of designated branch circuits.
In industrial applications, the line or phase bus bars are generally very large in order to accommodate very high levels of current. Typically, current transformers are coupled to the line or phase bus bars in order to step down the current and to enable monitoring equipment to be connected to the system. The line or phase bus bars, which may be up to about eight inches in diameter, may carry a current from 50 amps to 6,000 amps. The output, or secondary, of the current transformers typically comprise standard 16 gauge electrical wires and carry a standard current of 5 amps. To add an arcing fault detection capability to the system in the manner known in the prior art, the bus bar must be dismantled in order to place an arcing fault sensor around the bus bar. The cost of dismantling the bus bar and providing a sensor large enough to encircle the bus bar can be prohibitive.
The present invention is directed to an arcing fault detection system adapted for industrial applications which may be installed much less expensively than prior art systems.
In accordance with one aspect of the present invention, there is provided a system for detecting arcing faults in an electrical distribution system including a plurality of phase lines for connecting a power source to a respective plurality of loads, and a current transformer electrically coupled to one of the phase lines. The current transformer has a secondary line extending therefrom carrying an electrical current. The system monitors the secondary line for the occurrence of an arcing fault in the corresponding phase line and disconnects the load from the power source in response to the detection of an arcing fault in the corresponding phase line. The monitoring of the secondary line for the occurrence of an arcing fault comprises monitoring changes in current (or voltage) associated with the secondary line and producing a rate-of-change signal representing the rate of change of current (or voltage) associated with the secondary line. An arc-fault-detection signal is generated in response to variations in the rate-of-change signal representing an arcing fault.